As noted here, a law forbidding retired military officer from serving as Secretary of Defense until seven years after retirement currently stands in the way of the Mattis nomination, and will require fresh legislation to overcome. Now the path towards obtaining that legislation is beginning to come into focus.
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Over at War on the Rocks, I have a post explaining that a new statute will be needed in order for General Mattis to serve as Secretary of Defense. It's been done once before, with George Marshall during the Truman Administration.